PREDICTING BEARING LIFE

In 1978, the AFBMA introduced separate life factors for reliability, manufacturing, and bearing operation. The 1990 ANSI/AFBMA standards incorporated bearing life factors of 2.2 and 1.0 for ball and roller bearings respectively. These factors are more conservative than the ASME factors, but are to be used with other factors obtained from bearing manufacturers.

The 1992 STLE life adjustment factors reflect a bearing life increase of over 60 times, compared to 1950 standards.

Figure B - Predicted and experimental life of 120-mm bore, angular-contact ball bearings made from VIM-VAR AISI M-50 steel.

Using the original ANSI/AFBMA standards gives very conservative life estimates for ball bearings, Figure B. Here, the bearing life can be underpredicted by a factor of 100. By contrast, the new STLE factors give much more accurate, though still conservative, results for today's bearings. This method generally underpredicts bearing life by a factor of 2 or 3.

General bearing life calculation method

The ANSI/AFBMA standards combine three life factors in calculating bearing life:

L_na = a₁a₂a₃L₁₀

where:
L_na = Adjusted life, hr
a₁ = Adjustment factor for bearing reliability
a₂ = Adjustment factor for bearing materials and processing
a₃ = Adjustment factor for bearing operating conditions
L₁₀ = Basic rating life, hr

The STLE calculation method uses the same formula, but with more detailed life factors, as described in STLE book. Here are the basic steps:

First list the bearing design parameters (type, size, dimensions, materials, melting process, and hardness), and operating conditions (load, speed, temperature, L₁₀ life, and type lubricant).
Determine the bearing life factors a₁, a₂, and for these parameters. The first bearing life factor, a1, depends only on the probability of survival that you select: at 90% probability, a₁ is 1.

Determining a₂ and a₃ is more involved. For a₂, you obtain life factors for manufacturing variables from STLE book, then combine them into one value. Life factor a₃, which applies to operating conditions, is determined in a similar manner.

Finally, enter the three bearing life factors, along with the L10 value into equation (1) to obtain the predicted bearing life.

Predicting Bearing Life